The present invention relates to a glow plug having a ceramic heater consisting of a resistive ceramic material and, more particularly, to a self-temperature-controlling ceramic heater type glow plug which can function as a high-speed heating type glow plug and improves heating characteristics to ensure sufficient durability and a long after glow time.
As a glow plug of this type, glow plugs having various types of structures have been conventionally known. Of these glow plugs, a ceramic heater type glow plug has attracted attention since it can function as a high-speed heating type glow plug. For example, as a ceramic heater type glow plug of this type, Japanese Patent Laid-Open No. 57-41523 discloses a glow plug using a ceramic heater in which a heating wire consisting of tungsten (W) or a rhenium (Re) alloy is embedded in an insulating ceramic material. This glow plug has a higher heat transmission efficiency than that of a conventionally used sheath type glow plug and can function as a high-speed heating type glow plug since its heating characteristics or temperature rise characteristics are improved. In the above ceramic heater, however, a speed of heating, i.e., a rate of increasing a temperature is limited due to a difference between linear expansion coefficients of the metal wire and the ceramic material. Therefore, this glow plug still has a problem to function as a high-speed heating type glow plug. This problem as a high-speed heating glow plug cannot be avoided since a material having a large positive resistance temperature coefficient such as tungsten is used as the heating wire.
As a ceramic heater described above, Japanese Utility Model Laid-Open No 62-148869 proposes a ceramic heater manufactured by integrally forming the entire body from the interior to the surface of the heater by a resistive ceramic material. According to such a ceramic heater, the heater itself consisting of the resistive ceramic material is powered to generate heat. Therefore, unlike the ceramic heater in which the metal heating wire or the like is embedded in the insulating ceramic material, this ceramic heater is excellent in durability and can function as a high-speed heating type since its heating characteristics are improved. SiAlON is known as the resistive ceramic material for use in the ceramic heater of this type because SiAlON having a specific resistance which largely varies when the content of titanium nitride as a resistivity imparting material is increased/decreased can be arbitrarily, selectively used as a resistive or insulating material.
In order to allow, however, a conventional glow plug using the ceramic heater consisting of the above resistive ceramic material to rapidly generate a heat so as to function as a high-speed heating type glow plug such that, for example, a temperature of about 800.degree. C. is obtained in about one second, a control means must be provided in a power supply circuit between the glow plug and a battery to flow a large current to improve temperature rise characteristics in an initial stage of power supply, thereby heating the glow plug up to a predetermined temperature, and to prevent an overheating state to obtain saturation at a predetermined temperature (e.g., 1,100.degree. C.). As a result, a circuit arrangement is complicated to increase the manufacturing cost.
In addition, if a battery voltage is directly applied to increase a temperature to a saturation temperature (e.g., 1,100.degree. C.), it takes about five seconds to obtain a heating temperature of, e.g., about 800.degree. C., thereby largely impairing the performance as a high-speed heating type glow plug. Therefore, a demand has arisen for a certain countermeasure against these problems.
Furthermore, in recent years, a strong demand has arisen for the glow plug of this type to adopt a so-called after glow system in which a power supply state is maintained with respect to a glow plug for a predetermined time period after start of an engine to smoothly and correctly perform combustion in the engine. In addition, the after glow time is required to be prolonged as long as possible. That is, control is sometimes performed in a glow plug so as to obtain sharp temperature rise characteristics, to obtain a predetermined saturation temperature (about 1,100.degree. C.) in about, e.g., ten seconds, and to reduce the saturation temperature to a predetermined temperature or less (e.g., 1,000.degree. C. or less), thereby performing power supply after start of an engine, i.e., so-called after glow. That is, in a cold district or the like, even after an engine is started, it takes a long time period to set the engine in a warmup. In addition, in this non-warmup state, noise is large during idling, or white smoke is produced or an engine is stopped due to incomplete combustion, thereby posing an exhaustion of white smoke or noise problem. The after glow system is required to prevent these problems. In order to perform power supply control during after glow, the ceramic heater consisting of the above resistive ceramic material is excellent in durability or the like to achieve its effect. In this case, however, the following problem arises.
That is, as the after glow time as described above, a long time period of, e.g., ten minutes or more is currently required. For this purpose, however, a complicated circuit arrangement capable of properly performing power supply control with respect to the ceramic heater must be provided in a power supply circuit connected to the glow plug, resulting in high manufacturing cost. Therefore, in a ceramic heater type glow plug of this type, a demand has arisen for a certain countermeasure which can solve all the above problems and can properly and reliably realize the function as a high-speed heating type glow plug and so-called overshoot characteristics or the like during after glow.